System and method for spelling recognition using speech and non-speech input

ABSTRACT

A system and method for non-speech input or keypad-aided word and spelling recognition is disclosed. The method includes generating an unweighted grammar, selecting a database of words, generating a weighted grammar using the unweighted grammar and a statistical letter model trained on the database of words, receiving speech from a user after receiving the non-speech input and after generating the weighted grammar, and performing automatic speech recognition on the speech and non-speech input using the weighted grammar. If a confidence is below a predetermined level, then the method includes receiving non-speech input from the user, disambiguating possible spellings by generating a letter lattice based on a user input modality, and constraining the letter lattice and generating a new letter string of possible word spellings until a letter string is correctly recognized.

PRIORITY INFORMATION

The present application is a continuation of U.S. patent application Ser. No.: 12/507,388, filed Jul. 22, 2009, which is a continuation of U.S. patent application Ser. No.: 10/894,201, filed Jul. 19, 2004, now U.S. Pat. No. 7,574,356. The contents of which are incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to recognition and more specifically to combining speech and non-speech input to improve spelling and speech recognition.

2. Introduction

Automatic speech recognition (ASR) systems that are being deployed today have the ability to handle a variety of user input. ASR systems are deployed, for example, in call-centers where a person may call in and communicate with the spoken dialog computer system using natural speech. A typical call-center transaction might begin with a fairly unconstrained natural language statement of the query followed by a system or user-initiated input of specific information such as account numbers, names, addresses, etc. A transaction is usually considered successful if each of the input items (fields) is correctly recognized via ASR, perhaps with repeated input or other forms of confirmation. This implies that each field has to be recognized very accurately for the overall transaction accuracy to be acceptable.

In order to achieve the desired accuracy, state-of-the-art ASR systems rely on a variety of domain constraints. For instance, the accuracy with which a 10-digit account number is recognized may be 90% using a digit-loop grammar but close to perfect when the grammar is constrained to produce an account number which is in an account-number database. Similarly, if one has access to a names directory and the user speaks a name in the directory, the performance of ASR systems is generally fairly good for reasonable size directories.

In some applications, the use of domain constraints is problematic. As an example, consider an application whose purpose is to enroll new users for a service. In this case, information such as the telephone number, name etc., need to be obtained without the aid of database constraints. One could still use priori constraints, such as a names directory that covers 90% of the US population according to the US Census data, to improve recognition accuracy. However, if the names distribution of the target population does not match the US Census distribution, the out-of-vocabulary (OOV) rate could be substantially higher than 10%.

Recognition of long digit-strings, names, spelling and the like over the telephone, whether human or machine, is inherently difficult. Humans recover from recognition errors through dialog. Such dialogs, which might involve a prompt to repeat a portion of the digit string or a particular letter in a name, have been implemented in ASR systems but with limited success. In the short-term, it appears that the best way to achieve very accurate recognition of difficult vocabularies such as letters and digits is to use to supplement voice with other input modalities such as keypads that produce touch-tones. The telephone keypad is designed for numeric entry and therefore is a natural backup modality for digit-string entry. However, the keypad is not as convenient for the entry of letter strings such as when names are spelled.

Cluster keyboards that partition the letters of the alphabet onto subset keys have been designed to facilitate accurate letter-string entry using keyboards. The letter ambiguity for each key-press in these keyboards is addressed by hypothesizing words in a dictionary that have the highest probability according to a language model. Such methods are effective, but they require the use of specialized keypads. If one is constrained to use the standard telephone keypad, one possibility is to use speech for disambiguation. A scheme for integrating keypad and speech input has been introduced recently but are not as successful as would be desired.

What is needed in the art is a system and method to obtain spelling recognition using information from keypad input and improved strategies for the combined use of the non-speech input such as telephone keypad input as well as voice for highly accurate recognition of spellings.

SUMMARY

Accurate recognition of spellings is necessary in many call-center applications. Recognition of spellings over the telephone is inherently a difficult task and achieving very low error rates, using automatic speech recognition, is difficult. Augmenting speech input with input from the telephone keypad or other non-speech input source can reduce the error rate significantly. The present disclosure presents a number of embodiments for combining the non-speech input and speech input. Experiments on a name entry task show that spellings can be recognized nearly perfectly using combined input, especially when a directory lookup is possible. The disclosure applies primarily to spelling scenarios but is also applicable in other, more standard speech recognition contexts.

The disclosure includes systems, methods and computer-readable media that perform the steps of automatic speech recognition and include a component for keypad or non-speech input. An example embodiment relates to a method for recognizing a combination of speech followed by keypad or non-speech input. The disclosure will apply to disambiguate received speech via additional non-speech input. The method includes receiving speech followed by a keypad sequence from a user, dynamically constructing an unweighted grammar permitting all letter sequences that map to the received non-speech input, constructing a weighted grammar using the unweighted grammar and a statistical letter model (such as an N-gram letter model) trained on domain data, receiving speech from the user associated with the non-speech input and recognizing the received speech and non-speech input using the constructed weighted grammar.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and other advantages and features of the disclosure can be obtained, a more particular description will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the disclosure and are not therefore to be considered to be limiting of its scope, the disclosure will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 illustrates a basic system;

FIG. 2 illustrates an unweighted grammar generated from alternate input;

FIG. 3 illustrates a method embodiment; and

FIG. 4 illustrates another method embodiment.

DETAILED DESCRIPTION

Various embodiments of the disclosure are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the disclosure.

There are a number of ways to improve the performance of spelling recognition using the constraints provided by non-speech input or keypad input. FIG. 1 illustrates one embodiment of the basic architecture 10 of the disclosure. The architecture includes a spoken dialog system 10 communicating via a network 10 with a user 20. The user has at least two means to communicate with the system 12. First, the user has a speech input means 16 to provide speech to the system 12. This may include such speech input means as a telephone, handheld computing device, cell phone, voice over IP communication, or any other means for providing a speech signal which may be transmitted to the system 12. Second, the user 20 has an alternate input means 18 which includes, by way of example, a keypad input, a touch-sensitive screen, a mouse-click and display, etc. The alternate input means may comprise any non-speech input mechanism that is physically attached to the speech input means 16. The alternate input means 18 may also be separate from the speech input means 16. The alternate input means 18 also communicates information over the network 14 to the spoken dialog system for processing along with the speech input to improve the recognition of the speech input.

As can be appreciated, this only provides a basic description of the architecture and any variety of communication architectures, both wired and wireless, may be employed to communicate the speech and alternate input to the system 12. For example, given the vehicle embodiment below, the network may comprise a network connecting speech and non-speech inputs and a computer in the vehicle that processes the input and provides responses.

In an example embodiment, the alternate input 18 is a telephone keypad. There are either 3 or 4 letters of the alphabet that are associated with each key on the telephone keypad. FIG. 3 illustrates a method aspect of this embodiment which includes obtaining non-speech input (such as a keypad sequence) from a user (302) and dynamically constructing a grammar (304) that permits all letter sequences that map to the given non-speech input. An example grammar is shown in FIG. 2 for the name “susan”. In this example, the grammar is unweighted. Continuing with the embodiment shown in FIG. 3, if one had access to spellings that characterize the domain, such as a directory of names for the name recognition task, one could use a statistical letter model such as, for example, an N-gram letter model L_(N) trained on this data, and construct a weighted grammar (306). K _(w) =K∩L _(N)   (1) where K is the unweighted keypad grammar, and ∩ is the intersection operation. Any suitable statistical letter model will be sufficient for the purposes of the present disclosure. In one embodiment, the statistical letter model is an N-gram model.

If the corpus on which L_(N) is estimated is large, an unsmoothed N-gram model (only those N-grams that appear in the training corpus are allowed) provides a significant advantage. Next, the system receives speech from the user associated with the non-speech input (308) and the system recognizes the received speech using the speech, received non-speech input and weighted grammar (310). As has been mentioned above, using the keypad sequence is one example of non-speech input and typically the speech input is received before the non-speech input.

Other contexts in which this disclosure may be employed are for providing alphanumeric account numbers. These are particularly difficult sounds to accurately recognize by a spoken dialog system. Other alphanumeric scenarios include where a person is in a vehicle and is interested in receiving directions or other information. ASR in a vehicle presents extra background noise that increases the difficulty of accurate recognition. The person may need to provide address information which includes a combination of words and numbers. In a vehicle, the non-speech input may be provided via a touch-sensitive screen viewable by the driver or passenger. As an address is spoken, a database of addresses/street names/city names and other related information may be used to recognize words spoken. On the screen, a short list of possible recognition answers may be presented and the person may be able to provide input to identify the correct word or numbers. An example will illustrate the operation.

A challenge exists where the person begins to say or spell a street name and there is a large list of possibilities to present. The lattice-based approach of the present disclosure enables a dynamic approach of reducing and narrowing the list of possibilities as more speech information is received.

Suppose the driver desires directions on his navigation system in a vehicle to 5110 Spencer Street. When the spoken dialog system is in the position of receiving the address to look up, the person states “five one one zero . . . ” When the word “five” is spoken, a large list of possible addresses beginning with “5” is on the possibilities list. As the other words are spoken, “one one” and so on, the lattice-based approach enables the constraints on the lattice to dynamically be applied to locate the most probable path through the lattice. On the display, as the short list is generated, the numbers appear on the screen. Suppose that the confidence level in recognizing the number “zero” was low and there was some confidence in another number being recognized, say “three” due to background noise. The display could show the following:

-   -   5110     -   5113

The user could then provide either speech or non-speech input to disambiguate between the two numbers. The non-speech input may come in the form of touching the touch-sensitive screen, or using buttons on the steering wheel. In this regard, it is known that steering wheel buttons may be used for controlling speed and radio functions. These buttons may also be utilized when in this mode to navigate and control the screen to identify recognition input. For example, the scan/search radio button could be used to indicate up or down on the short list of recognition options to identify the correct option easily with the least distraction for the user.

If the user is using a T9 keypad (telephone keypad), then the lattice-based approach may be used to disambiguate spellings without the need to press the same key numerous times. For example, the letter “I” may be obtained by pressing the “4” key three times. However, when spelling, the user may be able to only hit each key once for one of the three or four letters associated with the key. The system according to the present disclosure can dynamically identify lists of possibilities by using lattices to disambiguate the possible spellings of words.

In another aspect of the example above, a user could start with a street name. If the noise in the vehicle prevents the recognition of the name “Spencer”, the system may query the user for the first letter in the name of the street. The user can they say “S” or “S as in Sam” and the display can provide a short list of streets beginning with the letter “S”. If the user begins by providing the street name, then a database of all numbered addresses on that street may be used to improve the recognition of the address. If the number 5113 was not a house number on Spencer street, the confidence score for the number 5110 would be raised.

The basic approach towards receiving speech and non-speech input according to the present disclosure has many applications in scenarios like the vehicle/address scenario where a combination of speech input and non-speech input in the forms of touching a touch sensitive screen or manipulating multi-function buttons can provide an efficient and safe exchange of information between the person and the spoken dialog system.

The accuracy of recognizing the spelling, words or names using state-of-the-art ASR systems is reasonably good, especially if good language models (of letter sequences) are available. One aspect of the disclosure provides for performing spelling recognition using ASR alone in the first pass, and use non-speech input only when the ASR confidence is low. This way, the inconvenience of using non-speech data entry will be limited to those utterances that are poorly recognized for reasons such as the presence of background noise or unusual accents. This approach is shown in FIG. 4.

As shown in FIG. 4, this embodiment provides a method comprising performing spelling recognition via ASR and received speech from the user, the ASR being performed using the statistical letter model L_(N) (preferably an N-gram letter model) trained on domain data and producing a letter lattice R_(LN) (402). The system determines whether the ASR confidence level is below a predetermined level (404). If the ASR confidence level is not below the threshold (406), then the ASR process ends. If the ASR confidence level is below the threshold (408), then the user is then asked to input the letter string using non-speech means such as, for example, the keypad (system instructions: “1 press for each letter in the word” or “press the volume button on the steering wheel to navigate the list of street name”) and the system receives the non-speech input (410) and generates a constraint grammar K (412). The final result is the letter string r=bestpath (R _(LN) ∘I ∘K)   (2) where ∘ denotes the composition of finite-state transducers, and I is a transducer that eliminates silences and other filler words in the recognized output (418).

Each of the concepts described herein could be followed by a lookup in a database (of valid words, names, etc.) to find a valid letter sequence. The resulting letter string r _(D)=bestpath (R _(NC) ∘D)   (3)

where R_(NC) is the word lattice obtained by one of the processes described below without a database constraint and D is a finite state network that accepts only valid letter strings. Implementing database lookup as a separate step from speech recognition has the following advantages: (1) The complexity of the recognizer does not grow with the size of the database/directory; and (2) The vocabulary (allowed letter strings) as well as domain-dependent language models (such as frequency of requested names) could be updated independent of the recognizer, thereby simplifying service deployment.

Another option is the use of non-speech input to constrain only the first N letters. For long names or long street names, keying in all the letters may be too burdensome, but keying in only the first few may be considered acceptable. This provides a way to tradeoff accuracy for convenience, and combined with a database lookup is very effective.

One task mentioned above associated with the process of speech recognition is the recognition of spelled names. In applications where a directory is not available, a common solution is to attempt to cover as large a target population as possible, using a directory of names obtained from an independent source such as the Census or the Social Security Administration in the United States or a listing a street names from a city database. However, one cannot depend on the distribution of names in the target population matching the distribution of the general population of the country. Table 1 shows the out-of-vocabulary (OOV) rate of names taken from three tasks, an AT&T customer service task associated with open names, and two corporate directories containing about 50,000 unique names, for a range of vocabulary sizes.

The Census data indicates that 90,000 of the most frequent names cover about 90% of the U.S. population. Table 1 illustrates Out-of-vocabulary rates for test names taken from three tasks as a function of the size of a given directory. From Table 1, it is clear that the OOV rates can be significantly higher for a given task. The conclusion is that the vocabulary (grammar) of an ASR system designed to recognize names will need to be very large to keep OOV rates low. The performance of a state-of-the-art letter string recognizer, on a spelled-names task over the telephone, is shown in Table 2 which shows the performance of name recognition using a spelled name grammar.

TABLE 1 OOV-type (token) % Vocabular Task1 Task2 Task3 100K 14.7 16.1 17.9 200K  9.0 (11.2) 10.1 11.6 800K  3.5 (4.8)  3.7 (9.0)  4.1 (9.3) 1.6M  2.3 (2.9)  2.7 (6.5)  2.8 (6.5)

TABLE 2 Unique Names name letter rt factor 124K 92 98.2 0.08 1.6M 83 95.2 0.27

The grammar is constrained to produce only valid names. In experiments, the acoustic model was trained discriminatively on a collection of independent databases of letter string utterances collected over the telephone. All the test names were in-grammar The accuracy of name recognition, i.e., the letter string accuracy, is fairly good at 92% for a 124,000 vocabulary and degrades to 83% for a vocabulary of 1.6 million names. An accuracy of 83% for name recognition may be considered acceptable in many applications. However, if the name is just one field in a number of fields that need to be filled to complete a task, it may be necessary to operate at much lower error rates to maintain reasonable task completion rates. Another point to note from Table 2 is that the resource requirements (real-time factor on a Pentium desktop with a 1 GHz processor) increases significantly for large grammars.

There are many systems that allow spelling input using just the keypad. For example, schemes that attempt disambiguation by finding a match in a dictionary are suitable for limited vocabularies. As the size of the vocabulary grows, directory lookup often does not result in a unique entry. Table 3 shows the performance of name recognition using keypad input only. Each letter is input using 1 key-press.

TABLE 3 Names Keys Lookup LM Lookup 124K 99K 48% 93% (98.4% 1.6M 1.1M  4% 91% (97.8%

Table 3 shows the results of an experiment where a single key-press is used to enter a letter. A directory containing 124,000 names maps to about 99,000 unique key sequences. A given key sequence, corresponding to the spelling of a name, results in a unique name after lookup about 48% of the time. The test set of names is the same as the one used in the recognition experiment above. When the directory lookup results in multiple names that match the key sequence, some other mechanism is required to select a single name or generate an ordered set. In this experiment, a language model related to the frequency of names according to the U.S. Census is used to pick the name with the highest frequency of occurrence amongst the set of retrieved names. Since this names distribution of this test sample matches reasonably well with Census distribution, the accuracy of name recognition increases to 93%. For a directory of 1.6 million names, a name is uniquely retrieved only 4% of the time without a Census language model and 91% when the language model is invoked. The risk, however, is relatively high (accuracy could drop from 91% to 4%) when the language model does not match the test data.

The above discussion gives some characterization of the spelled name entry problem. It is clear that solution based on speech or keypad alone may not be acceptable for applications that require highly accurate name entry, given the current state of speech recognition.

The results of name recognition using keypad input to constrain the recognizer are shown in Table 4. Table 4 shows the performance of name recognition using combined keypad and speck input. K-∞ implies that the letter string for the complete name is entered using the keypad. K-N implies that only the first N letters are entered using the keypad. 4 g-uns means an unsmoothed 4-gram model of the letter sequences. Real-time factor (RTF) for K-∞ condition is 0.01. As constraints are relaxed, the recognizer becomes less efficient, and RTF increases to 0.07 for the K-1 condition.

The first option is to key in every letter in the name (K-∞) and speak the letters. Even with no lookup, the name can be retrieved with an accuracy of 90% and a letter accuracy of 98.4%. At this point, there are no task constraints built into the system. This accuracy can be improved further by using a task-dependent N-gram model, which in this case was trained on the 1.6 million list of names. It is quite interesting that 98% accuracy can be achieved with a vocabulary of about 1.6 million names. When a directory is used for lookup, name recognition is nearly perfect even for 1.6 million name directory.

If only the first three letters are entered using the keypad, again one key-press per letter, the accuracy of name recognition with no lookup drops to 66% with no language model and 84% with a 4-gram letter sequence model. Directory lookup improves the accuracy significantly to near perfect recognition. Even the entry of the first letter of the name yields accuracies that are much higher than a fully constrained ASR system (improvement from 84% to 94%) for the 1.6 M names directory.

As explained below, one could reverse the order of the keypad and speech input. The results are shown in Table 5. Table 5 shows the performance of name recognition using speech input first, followed by keypad entry. The real-time factor for this scheme is in the range 0.1-0.4 because the first-pass recognition is not constrained by keypad input. An unsmoothed 4-gram model is used in the first pass. The name accuracy is a modest 71%. This improves to 91% with a directory lookup for a directory size of 1.6 million. Keypad constraints applied in a second pass significantly improve performance For the (K-∞) case, the accuracy improves to 97%, roughly matching the accuracy of the system where speech input follows keypad input. The other numbers in Table 5 show that the order of speech and keypad input does not really matter and that the performance in either case is very good.

TABLE 4 Accuracy-name (letter) % System no lookup 124K lookup 1.6M K-∞ 90 (98.4) 100 (100) 100 (100) K-∞ -4grm- 98 (99.7) 100 (100)  99 (99.8) K-3 66 (93.3) 100 (100)  98 (99.5) K-3-4g-u 84 (96.6)  99 (99.8)  97 (99.4) K-1 56 (88.9)  97 (99.2)  94 (98.6) K-1-4g-u 76 (93.4)  94 (98.2)  93 (97.8)

TABLE 5 Accuracy-name (letter) System no lookup 1.6M lookup 4g-u 71 (92.3) 91 (97.6) 4g-u K-∞ 97 (99.5) 99 (99.8) 4g-u K-3 84 (96.8) 97 (99.4) 4g-u K-1 75 (94.2) 93 (97.8)

Recognition of spellings is a challenge for ASR systems as well as humans. The strategies that human listeners employ for spelling recognition and error corrections are very interactive and involve prompts for partial strings, disambiguation using familiar words, such as “S as in Sam,” etc. which are not easily implemented in current ASR systems or are not very effective with current technology. Keypad input may not be very natural in a spoken language system and the design of a user interface to incorporate keypad and speech may be a challenge. However, these experiments have demonstrated that keypad combined with speech can be extremely effective. A variety of embodiments are presented for combining speech and keypad input and these provide mechanisms for a tradeoff between accuracy and convenience.

An effective method of entering spellings over the telephone is disclosed that augments speech input with keypad input. A variety of different mechanisms for integrating the two modalities were presented and evaluated on a names task. The results show that letter strings can be recognized very accurately even without directory-based retrieval. When a directory is used for retrieval, name recognition is nearly perfect even for large directories.

Embodiments within the scope of the present disclosure may also include computer-readable media for carrying or having computer-executable instructions or data structures stored thereon. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code means in the form of computer-executable instructions or data structures. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or combination thereof) to a computer, the computer properly views the connection as a computer-readable medium. Thus, any such connection is properly termed a computer-readable medium. Combinations of the above should also be included within the scope of the computer-readable media.

Computer-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Computer-executable instructions also include program modules that are executed by computers in stand-alone or network environments. Generally, program modules include routines, programs, objects, components, and data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of the program code means for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps.

Those of skill in the art will appreciate that other embodiments of the disclosure may be practiced in network computing environments with many types of computer system configurations, including personal computers, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. Embodiments may also be practiced in distributed computing environments where tasks are performed by local and remote processing devices that are linked (either by hardwired links, wireless links, or by a combination thereof) through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices.

Although the above description may contain specific details, they should not be construed as limiting the claims in any way. For example, the alternate means of input 18 is not limited to a telephone keypad but may be any type of keypad or any non-speech input, such as a stylus on a touch-sensitive screen, a button on a vehicle steering wheel or on a computing device connected to the spoken dialog system via voice over IP. Other configurations of the described embodiments are part of the scope of this disclosure. 

1. A method comprising: receiving, via a processor, a speech input; constructing an unweighted grammar permitting all letter sequences that map to the speech input; generating keypad constraints using the unweighted grammar and a weighted statistical letter model trained on a database of words; receiving non-speech input constrained by the keypad constraints; and recognizing the speech input and the non-speech input using the unweighted grammar and the weighted statistical letter model.
 2. The method of claim 1, wherein the database of words is a domain of words related to the non-speech input.
 3. The method of claim 1, wherein the weighted statistical letter model is an N-gram letter model.
 4. The method of claim 3, wherein the N-gram letter model is unsmoothed.
 5. The method of claim 1, further comprising: generating a final letter string based on the database of words.
 6. The method of claim 5, wherein generating the final letter string based on the database of words further comprises using a finite state network that accepts only valid letter strings.
 7. The method of claim 1, wherein the non-speech input comprises a portion of a word.
 8. The method of claim 1, further comprising: if an automated speech recognition confidence is below a predetermined level, prompting the user to enter a first three or less letters of the speech input by using a keypad to yield the non-speech input.
 9. A system comprising: a processor; a non-transitory computer-readable storage medium having stored therein instructions which, when executed by the processor, cause the processor to perform a method comprising: constructing an unweighted grammar permitting all letter sequences that map to speech input; generating keypad constraints using the unweighted grammar and a weighted statistical letter model trained on a database of words; receiving non-speech input constrained by the keypad constraints; and recognizing the speech input and the non-speech input using the unweighted grammar and the weighted statistical letter model.
 10. The system of claim 9, wherein the database of words is a domain of words related to the non-speech input.
 11. The system of claim 9, wherein the weighted statistical letter model is an N-gram letter model.
 12. The system of claim 11, wherein the N-gram letter model is unsmoothed.
 13. The system of claim 9, the non-transitory computer-readable storage medium having stored therein further instructions which, when executed by the processor, cause the processor to perform a method further comprising: generating a final letter string based on the database of words.
 14. The system of claim 13, wherein the final letter string is generated based on the database of words via a finite state network that accepts only valid letter strings.
 15. A non-transitory computer-readable storage medium storing instructions which, when executed by a computing device, cause the computing device to perform a method comprising: receiving a speech input; constructing an unweighted grammar permitting all letter sequences that map to the speech input; generating keypad constraints using the unweighted grammar and a weighted statistical letter model trained on a database of words; receiving non-speech input constrained by the keypad constraints; and recognizing the speech input and the non-speech input using the unweighted grammar and the weighted statistical letter model.
 16. The non-transitory computer-readable storage medium of claim 15, wherein the database of words is a domain of words related to the non-speech input.
 17. The non-transitory computer-readable storage medium of claim 15, wherein the weighted statistical letter model is an N-gram letter model.
 18. The non-transitory computer-readable storage medium of claim 17, wherein the N-gram letter model is unsmoothed.
 19. The non-transitory computer-readable storage medium of claim 15, the instructions further comprising: controlling the processor to generate a final letter string based on the database of words.
 20. The non-transitory computer-readable storage medium of claim 19, wherein the final letter string based on the database of words further comprises using a finite state network that accepts only valid letter strings. 